The present invention concerns bearings for timepieces and more specifically of the type that absorb shocks. Designers of mechanical watches have long devised numerous devices for absorbing the shock energy resulting from the impact of a wheel arbor against a wall of the hole in the base block through which the arbor passes, which also allow temporary displacement of the pivot-shank before it is returned to its rest position under the action of a spring.
FIG. 1 illustrates a shock absorber device or shock absorber bearing 1 including a support 2. This support has a housing 3 in which is arranged a pivot system 4, the purpose of which is to absorb, at least in part, any shocks experienced by the balance staff 5.
Pivot system 4 includes resilient means 4a and a pivot module 4b. The resilient means take the form, in this example, of a membrane. These resilient means are in the form of a disc-shaped base including a lower face and an upper face and having a central orifice, the lower face being opposite to the bottom of the support, ie. to hole 6 through which the balance staff, ending in a pivot-shank 5a, passes. The pivot module is secured in the centre of this disc. This disc includes, at the periphery thereof, a peripheral rim 4c extending in an axial direction, i.e. in a direction tending to move away from the upper face. Preferably, this rim extends such that the surface of the plane horizontal to the disc increases as the height of the rim increases.
Pivot system 4 is placed on the bottom of the support and the rim of the resilient means rests, for example, on a protuberance 2a of the support as seen in FIG. 1.
This pivot system is made of plastic material so that it can be fabricated using injection moulding techniques.
However, a drawback of such a shock absorber system is that it is not shock resistant. Indeed, if the pivot does not break, the pivot marks the plastic. The marking of the plastic forming the pivot system is caused by the resilient portion whose Young's modulus increases upon impact. The Young's modulus is also known as the modulus of elasticity (generally expressed in GPa), and it characterizes the resistance of a material to deformation.
Thus, as the Young's modulus increases, so the stress necessary for deformation increases. Consequently, the resistance of the resilient means of the pivot system which is opposed to the pivot increases and so the force between the pivot and the bearing increases. This increase in force for a very short time period may cause local plastic deformation to occur. This deformation can then cause malfunctioning of the shock absorber bearing.